The demand for enantiomerically pure compounds has grown rapidly in recent years. One important use for such chiral, non-racemic compounds is as intermediates for synthesis in the pharmaceutical industry. For instance, it has become increasingly clear that enantiomerically pure drugs have many advantages over racemic drug mixtures. The advantages of enantiomerically pure compounds (reviewed in, e.g., Stinson, S. C., Chem Eng News, Sep. 28, 1992, pp. 46-79) include fewer side effects and greater potency in many cases.
Traditional methods of organic synthesis have often been optimized for the production of racemic materials. The production of enantiomerically pure material has historically been achieved in one of two ways: the use of enantiomerically pure starting materials derived from natural sources (the so-called "chiral pool"); or the resolution of racemic mixtures by classical techniques. Each of these methods has serious drawbacks, however. The chiral pool is limited to compounds found in nature, so only certain structures and absolute configurations are readily available. Resolution of racemates often requires the use of resolving agents; this process may be inconvenient and is certain to be time-consuming. Furthermore, resolution often means that the undesired enantiomer is discarded, thereby wasting half of the material.
Cycloaddition reactions are powerful, frequently-exploited elements of the palette of transformations available to the synthetic organic chemist. There are numerous reasons for the importance of cycloaddition reactions, inter alia: 1) they are concerted reactions; 2) their products are significantly more complex than the required starting materials; 3) the relative simplicity and synthetic accessibility of the required starting materials; and 4) they are capable of generating a number of stereocenters in a single operation. The first of these points is tremendously important because concerted reactions transmit to their products in well-understood ways the stereochemical information contained in their starting materials.
The synthetic utility of cycloaddition reactions in which one of the reactants is a carbonyl group or analogue thereof--termed "Hetero"-cycloadditions --has been further expanded by progress in the development of asymmetric catalysts for these reactions. The Hetero-Diels-Alder reaction is perhaps the best example of a cycloaddition reaction whose utility has been has been augmented by research directed at the development of asymmetric catalysts (for a review, see: Danishefsky Chemtracts: Organic Chemistry 1989, 273). Catalysts comprising a transition metal ion and a chiral, non-racemic ligand have been reported to render enantioselective various Hetero-Diels-Alder cycloadditions; these reactions gave products in good to excellent enantiomeric excess (for leading references, see: Danishefsky and DeNinno, Angew. Chim., Intl. Ed. Engl. 1987, 26, 15-23; Corey and Loh, J. Am. Chem. Soc., 1991, 113, 8966-8967; Yamamoto et al., J. Org Chem., 1992, 57, 1951-1952; Keck et al., J. Org Chem., 1995, 60, 5998-5999; and Ghosh et al., Tetrahedron Lett. 1997, 38, 2427-2430).
The cyclohexene ring generated in a Diels-Alder reaction can be incorporated without further modification into biologically-active natural products, drug candidates, and pharmaceuticals. Additionally, the newly-formed cyclohexene ring may serve as a starting point for further synthetic transformations. For example, the A, B, and C rings of the steroid skeleton are functionalized cyclohexane rings; a number of routes to steriods based on the Diels-Alder reaction have been reported. The olefin in the cyclohexene derived from a Diels-Alder reaction can serve as a functional handle for subsequent transformations. The Diels-Alder reaction tolerates a wide range of "spectator" functionality--functionality not involved in, or affected by, the reaction conditions--which can serve as reactive sites in subsequent transformations. Finally, the so-called Hetero-Diels-Alder reaction provides access to unsaturated six-membered heterocycles.